I have the T4 on my 520SX. It lets the bike rev faster. It feels quite a bit more open.

At first I didn't care for it over stock but after riding with it a month or so I'm reconsidering. The cap that came with it has a 2" open. I just got the 1-1/2" end cap but will have to wait till Sunday to try it out.
The stock opening looks to be about an 1-1/4 to an 1-1/2. I'm guessing that the smaller (torque) endcap might make it really work nice. I'll post back after Sunday.

The PRO CIRCUIT T4 Exhaust works great on the ktm 400. It will work even better if you cut off about 1 inch and a half of the end of the exhaust. It will give the 400 more low to mid hit. Also a P-38( carb part) by Factory R&D will help give the motor more snap, wich it needs. You need to run also like a 52 rear sprocket. If you want to make the bike turn put 16mm Factory R&D Triple clamps on the bike and have Shock Therapy do your suspension. If you do all this you will have a bike ready to take on all of the big blue 426.

I believe the t4 comes with a 2" opening end cap. Is this the cap you are using? Also how did you come up with the idea to shorten the silencer? I've done this on 2-strokes before...is the concept the same on the 4-strokes. What about jetting?

Coach, I havev the large open cap on my pipe, and it works the best. It lets the bike rev more freely. The idea with the shortening of the silencer came when Pro Circuit was doing R&D on their pipes, and had Craig Davis (local Pro mx'er, and my good friend) testing pipes for them. The shorter pipe worked best on the 400, but it wasn't really necessary to do to the 520 (since it makes so much power anyway) and the fact that the shorter
the pipe, the louder it gets. It's great for racing, but not exactly eco-friendly. I shortened my pipe, and I love it. By the way, the last post was by Craig davis, as he was on my computer last night using my password. Have fun! Erik